The treatment of a number of diseases including hemoglobinopathies and malaria will require a fundamental understanding of both cellular and molecular aspects of human erythropoiesis. Many experimental methodologies aimed at understanding this process are inherently limited by the use of nonhuman cells and cell lines derived from transformed cells. The prospective study of human erythropoiesis using peripheral blood derived cells has been limited by retrospective analyses associated with semisolid culture of individual cells and contaminating populations within bulk cultures. We have taken a direct approach toward the prospective study of the early and late transcriptional events that encompass human erythropoiesis by examining cells that proliferate in culture specifically in response to the hormone erythropoietin. Using flow cytometry to analyze liquid cultured blood from normal volunteers, we have identified and temporally phenotyped the erythroid continuum of cells present in bulk cultures. This approach permitted us to identify, quantitate and purify erythroblasts that are transcriptionally committed to erythroid differentiation at early (progenitor) and late (precursor) developmental stages. The progenitor cell population is pivitol for the study of proliferation and differentiation events associated with normal and abnormal human erythropoiesis. The precursor cell population is equally important for defining the transcriptional events required for terminal erythroid cell differentiation. Our goal is to fully characterize these cell populations using current molecular genetic methods in order to understand and manipulate their transcriptional patterns. We are amassing a robust database describing the transcriptional profile of genes active in pure populations of erythroid progenitor and precursor cells. Several thousand genes have been identified including several hundred novel transcripts. These "normal" transcriptional profiles will be used as a template for disease correlates. Directly related studies involving the design and production of gene transfer vectors targeted to human hematopoietic cells are also being performed in the laboratory.